Conventionally, this kind of speed-changing device is well-known and it has a plurality of recesses at one of the outer periphery of a boss of the lever supported to the fixing member and the fixing member opposite to the outer periphery, and a ball engageable with each recess at the other, the ball being biased by a leaf spring toward each recess and engaging therewith to position the lever at a desired speed change stage and simultaneously maintain it thereat.
In such construction, the ball, when engaging with the recess, generates a sound which is too weak for a driver to accurately hear because the ball and recesses are provided between the opposite surfaces of the fixing member and the boss of operating lever, and are open to the exterior. Hence, the driver cannot accurately audibly check a speed change to a proper stage, causing him to rely on a visual check or on his sensing a switching condition of a drive chain to a desired one of multistage sprockets.
However, the visual check of the speed change stage cause the driver to divert his attention to watching where he is going which is dangerous. The sensing of a chain switching condition during pedaling is accurately performed by a skilled driver, but not by an unskilled one.
Since the aforesaid positioning mechanism in the conventional device is provided between the outer periphery of the boss and the fixing member opposite thereto, the fixing member should project in part outwardly with respect to the boss, which gives the device a bad appearance and enlarges it in size thereby subjecting it to more air resistance.